Do X-ray comets shed carbon?

17 Jun 2002

When Comet Hyakutake passed close to Earth in late March, it amazed astronomers by emitting intense X-rays. Now two British astronomers claim that this radiation came from the Sun and was deflected towards the Earth by tiny grains of organic material released from the comet's surface.

The mystery came to light when astronomers trained the German-American ROSAT satellite on Hyakutake. One theory was that the X-rays came from gas heated by the collision of debris from the comet with the solar wind. Another posited that solar X-rays were absorbed by material released from the comet's surface, then re-emitted as X-ray "fluorescence."

But Chandra Wickramasinghe of the University of Wales in Cardiff and his colleague Fred Hoyle say these processes would create barely 1 per cent of the X-rays observed. They claim ROSAT saw X-rays that came from the Sun's outer atmosphere, or corona, which were redirected by grains of carbon-based material measuring only a few nanometres across. "The observation makes sense if the comet contained huge numbers of virus-sized particles," says Wickramasinghe. Particles this small have not yet been identified in any comet. But Wickramasinghe says the Soviet Vega 1 and Vega 2 space probes provided tantalizing hints that such tiny particles might exist when they flew by Comet Halley in 1986.